Dual limit comparator with LM709

This voltage comparator circuit gives a positive output when the input voltage is a positive or negative more than 8.5V.

But in between that the output is negative voltage.
The positive limit point is defined by the ratio of R1, R2, and the negative point by R1, R3.
The forward voltage drop across the diodes must be allowed from it before.
The output may be inverted by reversing the inputs to the operational amplifier IC1, that LM709 is used without frequency offset.

12V Battery charger using UJT

This is an automatic 12V battery charger circuit, they uses the power SCR and UJT are main parts.
And they will not work except the battery 12V to be charged with right polarity, so prevent errors as well. The battery voltage controls the charger and when the battery is fully charged, the charger will not supply current to the battery anymore.

The battery charging current through the SCR MCR3818, when it is triggered into the conducting state by the UJT 2N4851 relaxation oscillator. And the oscillator is only activated when the battery voltage is low. Vb2b1 of the UJT derive the battery voltage that to be charged, and since Vp = Vd = Vb2b1 ; the higher Vb2b1, the higher Vp. When Vp exceeds the breakdown voltage of the zener diode ZD1, the UJT will stop to work and the SCR1 will not conduct.
T1 =Primary – 30 turn #22 , Secondary – 45 turns #22
R1 = Series resistance to limit current through SCR so use MCR3818 at 20 amps.
The setting VR1, we can charge the battery as needed.

Simple Smart Nicad (NiCd) Battery Charger

In application that demand for high current, Nicad (NiCd) rechargeable battery still has the unbeatable performance compared to NiMH and Lithium battery. The charger circuit here is smart in the way it handle the wrong polarity of the battery placement.

smart nicd circuit schematic

The core of this battery charger
circuit is the Q2, configured a constant current source for the
battery. LED D3 and zener diode D2 make the voltage applied to Q2 base
constant, therefore the voltage and the current across R1 is constant.
Using the value shown in the schematic diagram, the current flow through
R1 will be constant at around 50 mA.

You can see that the base of Q2 is
supplied by R3, which is controlled by Q1. Look at the Q1 base, it’s
controlled by the battery voltage, so Q1 won’t works if the battery is
placed in wrong polarity. Just note that although this battery charger
could charge four Nicad battery (in series), it can’t detect if one
battery placed in wrong polarity while two or three other batteries are
in right polarity. A small transformer, a bridge diode, and an
electrolytic capacitor is needed to supply this circuit. Remember to
provide a sufficient heat sink for Q2. This circuit will work as long as
the battery is not completely discharged, because its weak voltage is
needed to trigger the Q1 transistor.

Blackbox Sustain, An Audio Effect for Electric Guitar

Sustain effect in electric guitar is similar with compressor effect. The purpose of this effect is to make the note last longer when the string is plucked. The mechanism is actually work by controlling an amplifier gain, where the gain is automatically increased when the note fade out, therefore the note will be sustained to be heard until the actual sound almost completely gone.

blackbox sustain circuit schematic

You need a red LED and a small LDR
(light dependent resistor) to be assembled and protected in a lightproof
case. Select a small LDR and Red LED, set their layout that the LED ray
directly expose the sensitive surface
of the LDR, protect the LDR and the LED with lightproof glue, case, or
anything can be used to avoid the LDR receiving any light other than
from the LED. The LDR will be about 100-200 K ohm in dark condition and
about 1-5K? in day light exposure. You can use BC549 transistor type if
you can’t find BC209 type in your local store.

10 Watt Car Audio Amplifier

TDA2003 is my favorite audio amplifier integrated circuit chip in 10 Watt class. All you need is just adding few passive components and your amplifier will be ready. You can even amplify ultrasonic range if you wish to abuse its usage, just to convince you that
this chip is more than enough to handle any range of audio signal.
Although many manufacturer produce this TDA2003 chip, in general, this
various chip from various manufacturer normally comply with these
following features: Short circuit
protection between all pins, High current output ( up to 3 A), Built-in
Over temperature protection, and Low harmonic and crossover distortion.

tda2003 amplifier circuit schematic

Figure 1. Typical TDA2003 Audio Amplifier Application

tda2003 amplifier bridge circuit schematic

Figure 2. TDA2003 Audio Amplifier in Bridge Configuration

Figure 1 Show the circuit schematic diagram for typical application, can be operated for 12-18 DC power supply. Figure 2 show the application in bridge mode, giving more output to the loud speaker.